David Cameron has demanded an explanation for claims a police officer deliberately set out “to blacken the name” of Sutton Coldfield MP Andrew Mitchell.

But West Midlands Police Federation has refused to apologise for calling for him to resign when the 'plebgate' story first broke.

The dramatic development came after new evidence cast doubt on reports about his alleged clash with Downing Street cops.

Mr Mitchell resigned as Chief Whip in October over claims, which he has always denied, that he called police officers “plebs” and told them to “learn your f****** place”.

But fresh evidence has cast doubt on the allegations.

Mr Mitchell, 56, has always admitted swearing after police refused to open the main gate of Downing Street so that he could cycle out.

However, claims that he also made snobbish and disparaging comments were apparently confirmed in a police log of the incident, which was leaked to a newspaper.

This included references to “several members of the public” who had been standing outside the gate and “looked visibly shocked” as they heard Mr Mitchell abuse police officers.

But CCTV footage appears to show there was no-one standing by the gate at the time, suggesting at least one of the claims in the police log was wrong.

Furthermore, it has emerged Conservative MP John Randall received two e-mails apparently sent by a member of the public who witnessed the incident - but these were in fact sent by a police officer who was not even there at the time.

Mr Randall passed the e-mails on to Downing Street, and they appeared to offer independent verification that the claims against Mr Mitchell were true, shaking David Cameron’s confidence in his Chief Whip.

Some Conservatives have suggested Mr Mitchell should return to Government. It is unlikely he would be offered or accept the role of Chief Whip, but he could return to the role of International Development Secretary, which he held for two and a half years after the 2010 general election.

Mr Cameron told the House of Commons: “A police officer posing as a member of the public and sending an email potentially to blacken the name of a Cabinet minister is a very serious issue and does need to be seriously investigated.

“The Metropolitan Police Service is conducting a thorough and well-resourced investigation to get to the truth of this matter as quickly as possible.

“The Independent Police Complaints Commission will be supervising the investigation and I think we should allow them to get to the truth.”

Meanwhile, police bosses in the West Midlands have refused to apologise to Mr Mitchell following the latest twist in the ‘plebgate’ row.

Andrew Mitchell

The police federations for the West Midlands, West Mercia and Warwickshire forces had called for his resignation after the alleged Downing Street spat was reported in September.

They declined to comment on the latest developments and referred the Mail to a statement from The Police Federation of England and Wales.

The statement said: “We are concerned to hear of the arrest of a serving police officer and fully support the subsequent investigation into this.

“However, this remains a separate issue to the original claims made by Metropolitan police officers as to their treatment by a serving MP.

“It is disappointing that all information and evidence regarding the original incident was not revealed at the time - if that had occurred perhaps we would not be in the situation we now find ourselves in as a police service.

“The police federation is committed to fairness and supports its officers when their integrity is questioned without evidence to the contrary.